Although erythrocytic mannose-6-phosphate isomerase (MPI) has been reported to be undetectable in mammals, we have demonstrated that sufficient activity is present in some species to enable its electrophoretic analysis. A survey of MPI from 2656 baboons revealed four allelic isozymes which segregated codominantly in pedigreed families. The gene frequencies differed significantly among five subspecies of baboons. However, the MPI*C allele had the highest frequency in all subspecies, ranging from 0.830 to 1.000. MPI phenotypes from erythrocytes and liver of the same individual were identical, indicating that the erythrocytic enzyme is specified by the same gene locus as the liver enzyme.